Ducting extractor hob on a kitchen island to outside wall

[CJER]

We are currently renovating our ground floor and adding an extension on.

 

We currently have a thick concrete sub floor in the existing house as was built in the 80's, with no insulation, that we are taking down to then build back up, adding celotex 90mm and 65mm screed with UFH too which will match the new extension that the room will be part of.

 

We have been advised its best to duct the hob fumes/moisture out of the house, rather than recirculating, due to also having a wood log burner in the room and a MVHR, but have seen a lot of reading around the best way to do it.

 

My two main questions are (but will take any extra advice!)l

 

1. What happens to the moisture that condenses and can 'pool' in the duct if we're going vertically down through the island, through the floor under the island (beneath screed and insulation), underneath the floor to the external wall (approx 3 metres from duct entry) and vertically back up and out of the external wall?

2. Does the duct run under block wall and up through the cavity (then out the brick wall below DPC) or behind what will be tall kitchen cabinets?

[Conor]

Recirculating kit if you have an MVHR. I don't know who has been advising you, but they would be against the current consensus. 

 

For the floor, put in 100mm insulation with 50mm liquid screed. I'd say 100mm sheets are easier to come by and cheaper than 90mm anyway.

 

I assume the wood burner is room sealed with it's own air supply?

Edited Wednesday at 08:19 by Conor

[CJER]

We were told the MVHR (Venti Fluxo) was to be by the log burner on the opposite side of the room just to recirculate air and if it needs to draw from the room, also a generally good feature to recirculate what will be a well used room.

 

This was on the basis of the log burner being under 5kw without air supply, but since meeting with a HETAS supplier and fitter for the burner, they said they would fit it with its own air supply due to the room having an extractor of some sort (hob) anyway, not sure if this then makes the venti fluxo redundant and unnecessary?

 

The concern with a recirculating was it just isnt as efficient removing grease/moisture as extracting to outside.

Edited 21 hours ago by Nickfromwales

[Nestor]

@Conor is correct.

We have MVHR in an airtight* new build. It's great, works a treat in the background.

Just the extract and supply in a 60sqm kitchen / dining room, again not an issue.

Personally, If I did it again I would fit kitchen extract to vent outside but inaccessible ducting could be an issue. 

Recirculating hob should be enough and wood burner needs its own sealed air supply.

* Draughty catflap.

 

 

[dpmiller]

regardless of it being room-sealed I don't believe you can have a woodstove in a room which has an extractor of any kind. Read the B-Regs very closely for your jurisdiction... 

[JohnMo]

This sounds like a can of worms. 

 

If you have MVHR you really need external only air supply to stove.

 

As mentioned extract in same room isn't allowed. Extracting hood will just lead to room depressurised and the stove spilling combustion air into the room as soon as door is opened. Even if you have hood in recirculation mode, you will still have kitchen extract for the MVHR.

11 hours ago, CJER said:

The concern with a recirculating was it just isnt as efficient removing grease/moisture as a recirculating

Don't be concerned, I rarely even use our cooker hood - MVHR does the job 99.5% of the time. 

 

Your issue isn't the cooker hood it's the fire.

11 hours ago, CJER said:

MVHR (Venti Fluxo)

What is a ventilation fluxo you refer too?

 

11 hours ago, CJER said:

HETAS supplier and fitter for the burner, they said they would fit it with its own air supply due to the room having an extractor of some sort (hob) anyway, not sure if this then makes the venti fluxo redundant and unnecessary?

The air supply to stove is through wall so takes outside air to use as primary and secondary for combustion. It ensures stove doesn't use room air for combustion. It doesn't remove any need for room ventilation.

[CJER]

9 hours ago, JohnMo said:

This sounds like a can of worms. 

 

If you have MVHR you really need external only air supply to stove.

 

As mentioned extract in same room isn't allowed. Extracting hood will just lead to room depressurised and the stove spilling combustion air into the room as soon as door is opened. Even if you have hood in recirculation mode, you will still have kitchen extract for the MVHR.

Don't be concerned, I rarely even use our cooker hood - MVHR does the job 99.5% of the time. 

 

Your issue isn't the cooker hood it's the fire.

What is a ventilation fluxo you refer too?

 

The air supply to stove is through wall so takes outside air to use as primary and secondary for combustion. It ensures stove doesn't use room air for combustion. It doesn't remove any need for room ventilation.

Thanks for all the responses, i wasn't sure which to reply to but re the above:

 

Our architect designed with building control sign off, for a Venti fluxo (MVHR), next to the log burner (link below), the supplier said when installing in a room with extraction they would add an air supply to the log burner itself so wasn't worried about the kitchen hob extractor. 

https://www.venti-group.com/product/fluxo-mvhr/

 

The above link for venti fluxo was also to address the concern of the room being depressurised.

 

I imagine we aren't the only people with a large open plan kitchen/diner lounge (approx 50m2) that have a kitchen and log burner in the same room?

[saveasteading]

On 11/03/2026 at 07:50, CJER said:

best to duct the hob fumes/moisture out of the house

We have decided to do this rather than recirculate to the room. Wastes heat in winter, but avoids overheating the room  in summer.

We are installing it  right now and realising issues.

The best size of duct is 140/150mm. Obv you can't have that.

It sits on the concrete with insulation around it. Will condensation form in the duct? I'd expect it to.

And will water get into the pipe from cleaning the hob? I'd think so.

So we are setting the duct on thin polystyrene. Also setting it to an outward slope so any water runs away and out.

We have the depth for this but that is unusual.

 

I'm going off this hob with extraction. It was never my choice, and we are realising these additional unpublicised issues as we go.

 

NB maybe a rectangular duct will work better for you. It will be less efficient fir airflow, but your distance is short, and if will fit more readily.

 

[CJER]

23 minutes ago, saveasteading said:

We have decided to do this rather than recirculate to the room. Wastes heat in winter, but avoids overheating the room  in summer.

We are installing it  right now and realising issues.

The best size of duct is 140/150mm. Obv you can't have that.

It sits on the concrete with insulation around it. Will condensation form in the duct? I'd expect it to.

And will water get into the pipe from cleaning the hob? I'd think so.

So we are setting the duct on thin polystyrene. Also setting it to an outward slope so any water runs away and out.

We have the depth for this but that is unusual.

 

I'm going off this hob with extraction. It was never my choice, and we are realising these additional unpublicised issues as we go.

 

NB maybe a rectangular duct will work better for you. It will be less efficient fir airflow, but your distance is short, and if will fit more readily.

 

I'm not sure what extractor hob you're using but we're looking at an AEG downward extractor hob and the product they sell for this hob itself is a rectangular 220x90 and then they also provide attachments i believe to take it to a circular opening if need be, if that makes sense? 

[saveasteading]

22 minutes ago, CJER said:

what extractor hob 

Exactly that.

A 100mm /110 round pipe will do but it is recommended (and my hydraulics training concurs) that the bigger pipe is better.

But in your case I'd continue with rectangular, on an outward  slope, even 1:150, and on an eps layer...even 15mm will make a big difference.

It simply struck me when looking at it.. steam goes in and will very soon condense.

 

Btw, money aside, look at the island extractor on show in Wickes. Push a button and it rises. Very nice.

And I was impressed they had it installed and working.

Contrast that with Howdens who have their hob hacked into a standard unit and no ducting installed even to look at.

[CJER]

39 minutes ago, saveasteading said:

Exactly that.

A 100mm /110 round pipe will do but it is recommended (and my hydraulics training concurs) that the bigger pipe is better.

But in your case I'd continue with rectangular, on an outward  slope, even 1:150, and on an eps layer...even 15mm will make a big difference.

It simply struck me when looking at it.. steam goes in and will very soon condense.

 

Btw, money aside, look at the island extractor on show in Wickes. Push a button and it rises. Very nice.

And I was impressed they had it installed and working.

Contrast that with Howdens who have their hob hacked into a standard unit and no ducting installed even to look at.

yes i agree i imagine as long as its on a slight downward slope rather than opposite, should eventually make its way out.

 

I've seen these, very smart! would be interesting to hear how you get on with your installation it sounds like you've got it right!

[JohnMo]

So the dMVHR you are proposing to use work in pairs, one extracts and the second one applies air, they do this for around 20 secs and then swop. The room should remain at a stable pressure, not negative or positive. So long as you have 2 working as a pair you are ok.

 

Hob extract, is the danger zone, they will pull huge amounts of air from the room. If you are quite airtight (you are installing MVHR, so should be) you will depressurise the house enough to overcome the flue effect of the stove and instead of leaking air in to fire and flue seals, it will leak combustion smoke and CO into room. CO is a silent killer.

[CJER]

1 hour ago, JohnMo said:

So the dMVHR you are proposing to use work in pairs, one extracts and the second one applies air, they do this for around 20 secs and then swop. The room should remain at a stable pressure, not negative or positive. So long as you have 2 working as a pair you are ok.

 

Hob extract, is the danger zone, they will pull huge amounts of air from the room. If you are quite airtight (you are installing MVHR, so should be) you will depressurise the house enough to overcome the flue effect of the stove and instead of leaking air in to fire and flue seals, it will leak combustion smoke and CO into room. CO is a silent killer.

is this still the case even with the log burner having its own air supply? I guess what is the solution as we can't be the first people to have an open plan room with a log burner and extractor fan?

[JohnMo]

8 minutes ago, CJER said:

is this still the case even with the log burner having its own air supply? I guess what is the solution as we can't be the first people to have an open plan room with a log burner and extractor fan?

Yes

1. you have to open door to recharge the wood.

 

2. The flue generates a small negative pressure to draw air in, the external air supply gives air the aid combustion. If you room is a lower pressure than the flue, instead of combustion gases going up the flue, the room low pressure draws the combustion gases in to the room, the seals aren't fantastic on stoves or the flue.

 

How airtight is the house? Most people's houses leak air like sieve. Airtight and MVHR isn't the norm and you have design, not just add in airtight houses.

[CJER]

31 minutes ago, JohnMo said:

Yes

1. you have to open door to recharge the wood.

 

2. The flue generates a small negative pressure to draw air in, the external air supply gives air the aid combustion. If you room is a lower pressure than the flue, instead of combustion gases going up the flue, the room low pressure draws the combustion gases in to the room, the seals aren't fantastic on stoves or the flue.

 

How airtight is the house? Most people's houses leak air like sieve. Airtight and MVHR isn't the norm and you have design, not just add in airtight houses.

we're renovating a 1980's house that didnt have any insulation in the floor, so not particularly airtight or up to scratch as with new builds today, albeit, one half of the new extension will of course meet the new U ratings with wall insulation.

[JohnMo]

4 minutes ago, CJER said:

we're renovating a 1980's house that didnt have any insulation in the floor, so not particularly airtight or up to scratch as with new builds today, albeit, one half of the new extension will of course meet the new U ratings with wall insulation.

So why are you wasting money with MVHR? If you are not making effort to make airtight MVHR will just increase you ventilation rate.

 

I would dump the MVHR install humidity activated trickle vents in dry rooms and dMEV fans in wet.

 

But your issue still remains with extract and a closed stove in the same room - especially with the kitchen cooker hood that ventilated outside 

[CJER]

16 minutes ago, JohnMo said:

So why are you wasting money with MVHR? If you are not making effort to make airtight MVHR will just increase you ventilation rate.

 

I would dump the MVHR install humidity activated trickle vents in dry rooms and dMEV fans in wet.

 

But your issue still remains with extract and a closed stove in the same room - especially with the kitchen cooker hood that ventilated outside 

its not a case of not making effort, we are just going by architects plans, i've never heard of any of this before now so trying to absorb as much info as possible, especially when the building control inspector comes as well as log burner installer.

 

We do have 3 large velux's being installed in the pitched roof of the extension which have 'netatmo active' with them, which can activate opening and closing regarding temperatures/humidity, so i guess that could work and ditch the MVHR as you mentioned. 

[JohnMo]

The general consensus is airtightness better that 3m³/m² at 50Pa then MVHR can be cost effective, once installed. If your airtightness is worse than that it doesn't pay you have it.

 

The most silent dMEV fans I have found are Greenwood CV2 or CV3. The CV2 is generally available on eBay pretty cheap. With MVHR or dMEV you need to make sure your doors internally are uncut about 10mm to allow between room ventilation. All dry rooms need inlet ventilation for dMEV to function correctly.

 

If you have dMEV or MVHR then venting out cooker fan isn't needed. Just add a carbon filter and let it recycle the air. The MVHR or dMEV will get rid of any humidity, the grease and carbon filter will go the rest.

[Nickfromwales]

13 hours ago, CJER said:

Our architect designed with building control sign off, for a Venti fluxo (MVHR), next to the log burner (link below), the supplier said when installing in a room with extraction they would add an air supply to the log burner itself so wasn't worried about the kitchen hob extractor. 

https://www.venti-group.com/product/fluxo-mvhr/

 

The above link for venti fluxo was also to address the concern of the room being depressurised.

 

I imagine we aren't the only people with a large open plan kitchen/diner lounge (approx 50m2) that have a kitchen and log burner in the same room?

You're not alone in getting poor advice.

 

For someone to recommend what I assume is through wall MVHR in any house that's not designed to compliment or benefit from it, is just bonkers. This isn't a dig btw, just another example of the nuts advice handed out by 'professionals' who just don't have a single bloody clue. Borderline embarrassing for them tbh.

 

5 hours ago, CJER said:

its not a case of not making effort

Take the replies given here with a spoonful of sugar, we're a good crowd and are only well-intentioned  We just get frustrated seeing good people being given shit advice and having to pay the muppets for it.

 

Keep posting here, and you can get a good end result.

 

Whoever said that a through wall MVHR unit should replace the adventitious airflow to the WBS whilst your cooking is extracted needs a poke in the left eye, followed by one to the right. Such unqualified tripe is hard to hear without a coarse comeback, sorry.

[CJER]

16 hours ago, Nickfromwales said:

You're not alone in getting poor advice.

 

For someone to recommend what I assume is through wall MVHR in any house that's not designed to compliment or benefit from it, is just bonkers. This isn't a dig btw, just another example of the nuts advice handed out by 'professionals' who just don't have a single bloody clue. Borderline embarrassing for them tbh.

 

Take the replies given here with a spoonful of sugar, we're a good crowd and are only well-intentioned  We just get frustrated seeing good people being given shit advice and having to pay the muppets for it.

 

Keep posting here, and you can get a good end result.

 

Whoever said that a through wall MVHR unit should replace the adventitious airflow to the WBS whilst your cooking is extracted needs a poke in the left eye, followed by one to the right. Such unqualified tripe is hard to hear without a coarse comeback, sorry.

Hi Nick,

 

Appreciate your response, it has been a minefield, this is a drop in the ocean in terms of the advice we've received for our extension and ground floor renovation, many 3am nights researching things i've never known about, dont get me started on foundations and footings 😅

 

I'm thinking my best approach moving forward would be to get the hob ducted out as best practise to remove as much smell and cooking from the room if we have the opportunity to do so, and to then have the log burner installed with its own air supply (4.9kw wood burning stove), it's more of a feature and crutch to using heating than it is a necessity to warm the house full time.

 

We have a survey next week with the log burner supplier so perhaps in the space and looking at what it is we're doing in the flesh he'll be able to advise us further, albeit, I hope it wont be a case of...lets install, pay ££££, oh its failed spillage test, no cert!

 

Why is it the an MVHR isn't good, as in my understanding its also good to generally take old air and replace with new air, and if the log burner or hob did need more air, it can take from this...or does it not work like that?

[JohnMo]

56 minutes ago, CJER said:

Why is it the an MVHR isn't good,

 

Background to ventilation

 

Uncontrolled ventilation is leakage of the house structure - you can ot change it it's built into the building fabric.

 

Controlled ventilation is ventilation you can control, increase or decrease.

 

Basic, leaky house, no trickle vents are needed, an intermittent fan in bathroom and kitchen via a cooker hood. The combined uncontrolled leakage and a little controlled leakage gives satisfactory ventilation. Most your ventilation is uncontrollable leakage.

 

As you become more airtight you add trickle vents to dry rooms and either intermittent or permanent running fans to wet rooms, these again give ventilation, the tighter the building becomes the more you move towards permanent running fans. You still have a decent proportion of uncontrolled ventilation though.

 

MVHR needed for good air quality in sealed houses. But not the only solution. Here you almost have no uncontrolled ventilation.

 

MVHR in a leaky house. The house will still leak air in and out in an uncontrolled matter. MVHR will add extra ventilation on over and above the uncontrolled ventilation. Net result is way more ventilation than you need, and additional ventilation heat loss. Maybe too dry an atmosphere in winter leading to dry skin etc.

 

 

[CJER]

28 minutes ago, JohnMo said:

 

Background to ventilation

 

Uncontrolled ventilation is leakage of the house structure - you can ot change it it's built into the building fabric.

 

Controlled ventilation is ventilation you can control, increase or decrease.

 

Basic, leaky house, no trickle vents are needed, an intermittent fan in bathroom and kitchen via a cooker hood. The combined uncontrolled leakage and a little controlled leakage gives satisfactory ventilation. Most your ventilation is uncontrollable leakage.

 

As you become more airtight you add trickle vents to dry rooms and either intermittent or permanent running fans to wet rooms, these again give ventilation, the tighter the building becomes the more you move towards permanent running fans. You still have a decent proportion of uncontrolled ventilation though.

 

MVHR needed for good air quality in sealed houses. But not the only solution. Here you almost have no uncontrolled ventilation.

 

MVHR in a leaky house. The house will still leak air in and out in an uncontrolled matter. MVHR will add extra ventilation on over and above the uncontrolled ventilation. Net result is way more ventilation than you need, and additional ventilation heat loss. Maybe too dry an atmosphere in winter leading to dry skin etc.

 

 

ah this makes sense, again another minefield, i find it highly amusing/frustrating that we have been specified PIR and strong insulation in order to reach a certain U value, yet you walk a couple metres and the existing house doesnt have it, although i suppose its a good practise to move existing houses to being more efficient, but as you say, the MVHR is clearly overkill even with having a log burner and extractor fan. 

 

I think I'm reading it right!?

[saveasteading]

Update. The big outlet pipes for the island  hob fan are in, and laid to a gradient on eps.  There is a very noticeable draught at  the pipe , even though the building is a long way from enclosed. I was surprised at the amount of airflow but assume that the very strong wind is causing this. I have asked the family members who researched the hob, if the appliance is shuttered when not in use, or an always-open duct with a fan in it. We don't know.

An 8" hole in the wall would not be acceptable , but that is effectively what this is. I've suggested we put an offcut of plasterboard over the hob when not in use.... apparently that wont be aceptable.

OTOH are standard cooker hoods also open to the outdoors? I think so.

 

Re the stove. The Steading (complete)  has an ultra modern wbs, with direct air feed. It burns incredibly efficiently and there is no suggestion of smoke ever coming into the room rather than up the flue, or of any air leaks trough the door. I think you don't have to worry about interaction at all.

And I doubt that mvhr is appropriate either, based on the info provided. Do you mind saying the cost level? to that add for some builders' work in getting the ducts around the place.

[CJER]

3 minutes ago, saveasteading said:

There is a very noticeable draught at  the pipe

do you have a backdraft damper/non return valve on the opening on the outside wall? 

 

4 minutes ago, saveasteading said:

shuttered when not in use, or an always-open duct with a fan in it. We don't know.

i'm not sure what you mean by this?

 

4 minutes ago, saveasteading said:

An 8" hole in the wall would not be acceptable

is this in terms of regs or aesthetic?

 

5 minutes ago, saveasteading said:

Do you mind saying the cost level? to that add for some builders' work in getting the ducts around the place.

cost level of MVHR? I think 434.50 plus VAT - https://www.venti-group.com/product/fluxo-mvhr/

I'm not sure on the builder cost as the breakdown doesnt include the install of this separately on its own. i can ask him when we get to that point but it may be a while off, re messages above it seems MVHR isn't suitable, unless your house is a newer build or sealed tight?

[saveasteading]

5 minutes ago, CJER said:

on the outside wall? 

Hadn't thought of that.  yes we should do that. It will restrict flow to some extent but we must try.  But it would be best if the appliance has an automatic shutter, which is what I meant in your second query.

 

7 minutes ago, CJER said:

in terms of regs or aesthetic?

No, in terms of having a draughty hole in the room.

 

8 minutes ago, CJER said:

434.50 plus VAT

Wow. You won't get much kit for that.

Is that simply a fan unit with heat exchanger and no ducting? Ive seen them and expect that the expelled air is drawn straight back in and the intake air is sucked straight back out.